1. Field of the Invention
The present invention relates to a process for preparing higher oxo alcohols by two-stage hydrofornylation of olefin mixtures, which includes selective hydrogenation of the hydroformylation mixtures.
2. Description of the Background
It is known that higher alcohols, in particular those having from 6 to 25 carbon atoms, can be prepared by catalytic hydroformylation (or oxo reaction) of the olefins having one carbon atom less and subsequent catalytic hydrogenation of the aldehyde- and alcohol-containing reaction mixtures. The alcohols are predominantly used as starting materials for the preparation of plasticizers or detergents.
The type of catalyst system and the optimum reaction conditions for the hydroformylation are dependent on the reactivity of the olefin used. The dependence of the reactivity of olefins on their structure is described, for example, by J. Falbe, New Syntheses with Carbon Monoxide, Springer-Verlag, Berlin, Heidelberg, New York, 1980, pages 95 ff. The varying reactivity, especially of the isomeric octenes, is likewise known (B. L. Haymore, A. van Hasselt, R. Beck, Annals of the New York Acad. Sci., 415 (1983), pages 159-175).
Industrial olefin mixtures which are used as starting materials for the oxo synthesis comprise olefin isomers of the most varied structures having differing degrees of branching, differing position of the double bond and, in some cases, even differing carbon numbers. This applies especially to olefin mixtures which have been produced by dimerization, trimerization or further oligomerization of C.sub.2 -C.sub.5 olefins or other easily accessible higher olefins or by co-oligomerization of the olefins. As examples of typical isomeric olefin mixtures which can be reacted by rhodium-catalyzed, or preferably by cobalt-catalyzed, hydroformylation to give the corresponding aldehyde mixtures and alcohol mixtures, tripropenes and tetrapropenes and dibutenes, tributenes and tetrabutenes may be mentioned.
The rate of the hydroformylation reaction decreases with increasing carbon number and with the degree of branching. The reaction rate of linear olefins can be greater by a factor of 5 to 10 than that of the branched isomers. The position of the double bond in the olefin also influences the reactivity. Olefins having a terminal double bond react markedly more rapidly as compared to isomers having an internal double bond. Because of the differing reactivity of the olefin isomers, relatively long reaction times are required if it is desired to achieve the most substantial possible conversion of the olefins. However, as a result, the product yield is decreased due to unwanted side reactions and secondary reactions. This also occurs if attempts are made to shorten the reaction times by higher reaction temperatures. Especially because of the varying reactivity of the isomers, it is difficult to achieve high conversion rates and simultaneously high selectivities in the hydroformylation of olefin mixtures. This applies in particular to single-stage hydroformylations.
According to DE 32 32 557 A1, alcohols are prepared by a two-stage hydroformylation of monoolefins having from 3 to 20 carbon atoms. In the first reaction stage, the olefins are converted to the aldehyde, using a cobalt catalyst, with degrees of conversion of from 50 to 90%, the formation of alcohols being suppressed. The cobalt catalyst is then removed from the reaction mixture and this reaction mixture is hydroformylated again in a second stage using a cobalt organophosphine complex as catalyst. At the same time, the aldehyde formed in the first stage is hydrogenated to the alcohol. A disadvantage in this process is that, particularly in the second hydroformylation stage, a considerable part of the olefins is hydrogenated instead of being hydroformylated.
Accordingly, there remains a need for improved processes for preparing alcohols via the oxo reaction.